In a combined cycle plant and the like, used is an exhaust heat recovery boiler (sometimes referred to as HRSG) that obtains steam by heating water that flows in an evaporator arranged in a gas turbine exhaust gas passage using gas turbine exhaust, and in the HRSG, disposed is an exhaust smoke denitrating apparatus that, in order to eliminate nitrogen oxides in an exhaust gas, injects ammonia as a reducing agent for a denitration reaction from an ammonia injection unit at an upstream side of a disposing part of a denitration catalyst, sufficiently mixes the ammonia with the exhaust gas, and then causes a denitration reaction on the denitration catalyst.
In recent years, for greater ease in storage and handling than liquefied ammonia, ammonia water is used in many cases for the abovementioned ammonia injection unit, and as the device for evaporating the ammonia water, there is an electric heater evaporation method and an exhaust gas circulation type that extracts a part of the exhaust gas from the exhaust heat recovery boiler (HRSG) and uses the extracted exhaust gas for evaporation. From the viewpoint of improvement in plant efficiency, the exhaust gas circulation type that is relatively small in utility usage is often used.
The abovementioned exhaust gas circulating ammonia injection unit employs a method for extracting a high-temperature exhaust gas from the exhaust heat recovery boiler to heat ammonia water and supplying again the ammonia water as ammonia vapor into the exhaust gas passage.
In order to display the performance of the abovementioned denitration catalyst, the exhaust smoke denitrating apparatus is usually disposed in an exhaust gas temperature region of 300 to 350° C. This exhaust gas temperature region is an exhaust gas passage in an area where a high-temperature evaporator is disposed or on a rear stream side thereof. FIG. 6 shows a configuration example of a conventional exhaust smoke denitrating apparatus using an exhaust gas circulating ammonia injection unit applied to an exhaust heat recovery boiler.
Conventionally, a 300-350° C. exhaust gas extracted from an exhaust heat recovery boiler 1 is supplied through an exhaust gas supply pipe 6 to an ammonia injection unit 4, and after the pressure is raised by circulating fans 25 and 25′ so as to overcome a pressure loss of piping equipment, the exhaust gas has been sent to an ammonia water evaporator 10. Normally, a total of two circulating fans 25 and 25′ are used for a constant operation and a reserve, and heat resistance to 300 to 350° C. is required for both. Ammonia water regulated to a necessary flow by an ammonia injection control valve 11 is injected into the evaporator 10, and is heated by the exhaust gas inside the evaporator 10, so that an ammonia-containing vapor is obtained.
For evaporation of the ammonia water, the exhaust gas temperature at an outlet of the evaporator 10 is lowered to approximately 120 to 150° C., and this exhaust gas is injected by the ammonia injection unit 4, through an ammonia-containing gas supply pipe 13, to the upstream of a denitration catalyst layer 2 from an ammonia-containing gas injection nozzle 3 provided in a duct of the exhaust heat recovery boiler 1.
Patent Document 1: Japanese Published Unexamined Patent Application No. H07-16431